High strain rate deformation analysis of UFG aluminum sheet samples

The dynamic behavior of ultrafine-grained (UFG) and coarse-grained commercially pure aluminum was characterized in tension at a wide range of strain rates using the tensile Hopkinson Split Bar (THSB) technique and a servohydraulic materials testing machine. Sheet UFG aluminum specimens were wire EDM cut from large equal channel angular pressed (ECAP) ingots, manufactured using the route C+Bc. UFG aluminum is prone to necking at very low strain levels due to its inability to strain harden. Although necking commences almost immediately after yielding, the material is capable of deforming plastically to relatively large plastic strains before final failure. One problem with the tensile Hopkinson Split Bar method with dog-bone specimens is the inaccurate strain measurement due to the deformation taking place outside the specimen gauge section. Strain measurement inaccuracy can, however, be overcome by monitoring the deformation with the Digital Image Correlation (DIC) method, which gives the full field strain distribution of the sample surface. In this work, Aramis® 3D image correlation software with two digital high speed cameras was used for the full field strain measurements. Accurate strain measurement enables, among other things, determination of the strain rate distribution and dynamical necking behavior of the specimen. The UFG material is shown to strain harden during necking at high strain rates, although the strain hardening rate is not enough to postpone the necking at room temperature.